Preparing a Surface of a Sapphire Substrate for Fabricating Heterostructures

ABSTRACT

A method of fabricating a heterostructure comprising at least a first substrate ( 120 ) made of sapphire and a second substrate ( 110 ) made of a material having a coefficient of thermal expansion that is different from that of the first substrate. The method includes a step (S 6 ) of molecular bonding the second substrate ( 110 ) on the first substrate ( 120 ) made of sapphire. The method also includes, prior to bonding the two substrates together, a step (S 1 ) of stoving the first substrate ( 120 ) at a temperature that lies in the range 100° C. to 500° C.

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC 371 ofInternational Application PCT/EP2009/065202 filed on Nov. 16, 2009.

This Application claims the priority of French Application No. 08/57854filed Nov. 19, 2008, the entire content of which is hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to fabricating heterostructures formed bybonding at least one substrate made of a semiconductor material such assilicon on a substrate made of sapphire (Al₂O₃). The invention appliesin particular to fabricating silicon-on-sapphire (SOS) type structures.

BACKGROUND OF THE INVENTION

Heterostructures comprising a layer of silicon on a sapphire substratepresent particular advantages. SOS structures enable high frequencydevices to be made that present low energy consumption. The use ofsapphire substrates also makes it possible to achieve very good heatdissipation, better than that obtained for example with siliconsubstrates.

SOS structures were initially made by epitaxially growing a layer ofsilicon on a sapphire substrate. Nevertheless, with that technique, itis difficult to obtain layers or films of silicon that present a lowdensity of crystal defects, given the large difference between thelattice parameters and the coefficients of thermal expansion of the twomaterials.

In another technique, SOS structures are made by assembling a layer ofsilicon on a sapphire substrate. In well-known manner, use is made ofmolecular bonding (also known as “direct wafer bonding” or “fusionbonding”) which is a technique that enables two substrates to be bondedtogether providing they present surfaces that are perfectly plane(“mirror polish”), and without using an intermediate adhesive (glue,solder, etc.). Bonding is typically initiated by local application of asmall amount of pressure to the two substrates that have been put intointimate contact. A bonding wave then propagates over the entire extentof the substrate in a few seconds.

In addition, in order to enable good molecular bonding to be achievedbetween the substrates, their bonding faces need to present a very lowdensity of contaminants. These contaminants, which may come from thematerial itself or from prior treatment such aschemical-mechanical-polishing (CMP), are essentially of particulate,metallic, and organic origin.

Consequently, it is well known to proceed with cleaning of the polishedbonding surfaces of each of the substrates. With sapphire, the cleaninggenerally consists in treating the substrate with a chemical cleaningagent of the RCA type.

Furthermore, in order to obtain bonding energy between the twosubstrates that is sufficient to be able in particular to withstand thesubsequent steps of polishing, chemical attack, etc., the two substratesas bonded together in this way are subjected to heat treatment known asa bonding reinforcing anneal or as a stabilizing anneal. The anneal isgenerally performed at high temperatures of about 700° C. to 800° C.

Nevertheless, with a heterostructure made by bonding a silicon substrateon a sapphire substrate, such temperatures cannot be used because of thelarge difference between the coefficients of thermal expansion ofsilicon and of sapphire (3.6×10⁻⁶/° C. for silicon and 5×10⁻⁶/° C. forsapphire). If a silicon heterostructure on sapphire is raised afterbonding to the temperatures that are usually used for reinforcing thebonding interface, high thermomechanical stresses arise in thestructure, thereby leading to the appearance and propagation of cracksin the silicon.

Consequently, in order to preserve the integrity of the silicon, annealsfor reinforcing the bonding interface can be performed only attemperatures that are relatively low (<300° C.) compared with thoseusually used. This temperature limitation does not enable a high levelof bonding energy to be obtained between the silicon substrate and thesapphire substrate.

Methods of bonding silicon-on-sapphire are described in the followingdocuments:

G. P. Imthurn, G. A. Garcia, H. W. Walker, and L. Forbes, “Bondedsilicon-on-sapphire wafers and devices”, J. Appl. Phys., 72(6), Sep. 15,1992, pp. 2526-2527;

U.S. Pat. No. 5,441,591;

Takao Abe et al., “Dislocation-free silicon-on-sapphire by waferbonding”, January 1994, Jpn. J. Appl. Phys., Vol. 33, pp. 514-518;

Kopperschmidt et al., “High bond energy and thermomechanical stress insilicon-on-sapphire wafer bonding”, Appl. Phys. Lett., 70(22), p. 2972,1977.

SUMMARY OF THE INVENTION

One of the objects of the invention is to remedy the above-mentioneddrawbacks by proposing a solution that enables a heterostructure to beobtained by bonding, on a sapphire substrate, another substrate having acoefficient of thermal expansion that is different from that ofsapphire, and to do so while obtaining good bonding energy between thesubstrates and while limiting the appearance of defects after bondingand while limiting treatments after bonding.

To this end, one aspect of the present invention relates to a method offabricating a heterostructure comprising at least a first substrate madeof sapphire and a second substrate made of a material having acoefficient of thermal expansion that is different from that of thefirst substrate, the method including a step of molecular bonding thesecond substrate on the first substrate made of sapphire, in whichmethod, prior to bonding the two substrates together, a step isperformed of stoving the first substrate at a temperature lying in therange 100° C. to 500° C. When the stoving step is performed at atemperature of 100° C., the duration of the stoving step is then atleast 1 hour (h).

In unexpected manner, and as explained below in detail, such stoving ofthe sapphire substrate prior to bonding serves to improve significantlythe energy and the quality of the bonding compared with bondingperformed without the prior stoving step.

In an aspect of the invention, the stoving step is performed at atemperature of about 200° C. for a duration of about 2 h.

In another aspect of the invention, the quality of the bonding, and inparticular the bonding energy, may be further improved by activating thebonding surface(s) of one or both substrates by means of plasmatreatment.

To activate the bonding surface of the first substrate made of sapphire,the plasma is used at a mean power density that is preferably less thanor equal to 1 watt per square centimeter (W/cm²). The plasma is alsopreferably a plasma based on oxygen.

According to an embodiment of the invention, the method furtherincludes, prior to bonding the two substrates together, forming an oxidelayer on the bonding surface of the second substrate.

Molecular bonding between the first and second substrate is preferablyperformed at ambient temperature.

After the two substrates have been bonded together, the method mayfurther include a step of performing a bonding stabilizing anneal at atemperature of less than 300° C. This limit on the temperature of thestabilizing anneal serves to avoid excessive stresses arising in thestructure because of the difference between the coefficients of thermalexpansion of the two substrates. In spite of temperature being limitedin this way, the stoving step of the invention makes it possible toobtain good bonding energy.

The second substrate may in particular be constituted by a layer ofsilicon or by a silicon-on-insulator (SOI) structure.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention appear from thefollowing description of particular implementations of the inventiongiven as non-limiting examples, with reference to the accompanyingdrawings, in which:

FIG. 1 is a chart showing bonding energy values obtained as a functionof how the sapphire substrate surface is prepared and as a function ofthe stabilization anneal temperature;

FIG. 2 is a chart showing the different lengths of ring obtained as afunction of the mean power density of the plasma used for activating thebonding surface of the sapphire substrate;

FIGS. 3A to 3F are diagrammatic views showing the fabrication of aheterostructure by implementing a method of the invention;

FIG. 4 is a flow chart of the steps implemented while fabricating thethree-dimensional structure shown in FIGS. 3A to 3F; and

FIG. 5A shows an SOS type heterostructure in which the bonding surfaceof the sapphire support substrate has been prepared in accordance withthe prior art, while FIG. 5B shows an SOS type heterostructure with thebonding surface of the sapphire support substrate prepared in accordancewith an implementation of the method of the invention.

DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION

The present invention applies in general to molecular bonding between afirst substrate made of sapphire and a second substrate made of someother material that presents a different coefficient of thermalexpansion, such as in particular: silicon; quartz, germanium; andmaterials of the III-V group having a coefficient of thermal expansiongreater than that of silicon, such as GaAs or InP.

As is well known in itself, the principle of molecular bonding, alsoknown as direct bonding, is based on putting two surfaces into directcontact, i.e. without using any specific bonding material (adhesive,wax, solder, etc.). Such an operation requires the surfaces for bondingto be sufficiently smooth, free from particles or contamination, and tobe sufficiently close together to enable contact to be initiated,typically at a distance of less than a few nanometers. Under suchcircumstances, attractive forces between the two surfaces are highenough to cause molecular bonding to occur (bonding induced by all ofthe attractive forces (Van Der Waals forces) involving electroninteraction between atoms or molecules of the two surfaces for bondingtogether).

Nevertheless, when bonding a sapphire substrate with another substratehaving a coefficient of expansion that is different from that ofsapphire, the temperature of the reinforcing or stabilizing anneal mustbe limited (less than 300° C.) in order to avoid cracks appearing anddeveloping in the substrate bonded on the sapphire. Consequently, thebonding surfaces of the two substrates need to be prepared as well aspossible for enhancing molecular bonding and obtaining high bondingenergy.

As explained above, the sapphire substrate is cleaned after its bondingsurface has been polished, which is generally performed by CMP, apolishing or planarizing technique that is well known and that makes useof fabric associated with a polishing solution containing both an agentthat is suitable for attacking the surface of the layer chemically (e.g.NH₄OH) and abrasive particles suitable for attacking said surfacemechanically (e.g. particles of silica).

The bonding surface of the sapphire substrate is usually subjected toRCA type chemical cleaning which may be followed by scrubbing.

Nevertheless, the Applicant has observed that, even when the sapphiresubstrate is prepared in that way, the bonding of a silicon substrate ona sapphire substrate can give rise to results that are unsatisfactory,leading in particular to a high density of defects in the silicon, tothe formation of a ring (a non-bonded zone at the margins of the wafers)that is wide and irregular, and low bonding energy.

Unexpectedly, the Applicant has found that stoving the sapphiresubstrate prior to bonding enables the quality of the resulting bondingto be significantly improved compared with bonding performed withoutsuch stoving. FIG. 1 shows the bonding energy levels obtained as afunction of various different preparations of the bonding surface whenfabricating heterostructures of the silicon-on-sapphire (SOS) type. Itcan be seen that the bonding energy is greater when the sapphiresubstrate has previously been subjected to stoving at 200° C. for 2 hprior to cleaning and scrubbing (columns C), compared with RCA cleaningon its own (columns A), or with RCA cleaning followed by scrubbing(column B), and that this applies regardless of the stabilizing annealtemperature (lying in the range ambient temperature to 200° C.).

The Applicant has also measured the density of defects (for defects ofsize lying firstly in the range 100 micrometers (μm) to 500 μm, andsecondly in the range 5 μm to 100 μm) on a first SOS typeheterostructure for which fabrication included cleaning and scrubbingthe sapphire substrate, bonding a silicon substrate on the sapphiresubstrate, a stabilizing anneal of the bonding, and thinning of thesilicon substrate by mechanical polishing (grinding) and chemicaletching (TMAH), and on a second SOS type heterostructure for whichfabrication included all of the steps used for the first heterostructuretogether with an additional prior step of stoving the sapphiresubstrate. The second heterostructure presented a defect density thatwas ten or more times smaller than the density presented by the firstheterostructure. In addition, the second heterostructure presented ringtype margin defectuosity (non-transferred peripheral zone as shown inFIG. 2) that was divided by two compared with the first heterostructure.

The step of staving the sapphire substrate in accordance with theinvention is performed at a temperature lying in the range 100° C. to500° C. The duration of the stoving is a function of its temperature. Itlies between several minutes and several hours depending on thetemperature used. For stoving at the lowest temperature, i.e. 100° C.,stoving is performed for a duration of at least 1 h, and preferably overa duration lying in the range 4 h to 5 h. For a temperature of 200° C.,the duration of the stoving is about 2 h. At 500° C., the duration ofthe stoving lies in the range a few minutes to one hour. Consequently,the higher the staving temperature, the shorter its duration.

The stoving is performed in air or in an inert gas such as nitrogen orargon at normal pressure (i.e. atmospheric pressure).

The stoving of the invention serves to eliminate contamination oforganic origin in a manner that is much more effective than when usingchemical cleaning of the RCA type.

This stoving step also presents the advantage of not modifying thesurface state of the sapphire, i.e. of not creating additional atomicsteps (“miscut”). Contrary to heat treatment performed at hightemperature, stoving in accordance with the invention does not modifythe local surface of the sapphire wafer.

According to another aspect of the invention, the quality of thebonding, and in particular the bonding energy, can be further improvedby activating the bonding surface(s) of one or both substrates by meansof a plasma treatment.

Although activation by plasma treatment is well known for reinforcingbonding energy when performing molecular bonding, the Applicant hasdetermined conditions for such treatment in which optimum bonding energyis obtained while limiting any edge loss type margin defectuosity.

Thus, tests shown in FIG. 2 have shown that the value of the mean powerdensity of the plasma has an influence on the size of the ring(non-bonded zone at the margins of the substrates) and on post-bondingdefectuosity. The Applicant has found that in order to obtain goodactivation of the sapphire bonding surface while avoiding surfacedegradation that could lead to ring type margin defectuosity(non-transferred peripheral zone), the mean power density of the plasmaneeds to be limited to about 1 W/cm². This limit on the plasma powerdensity for optimizing bonding is unexpected in that as a general rulethe power density of the plasma is not limited to such a value when itis desired to maximize activation of the bonding surfaces.

The bonding surface of the sapphire substrate and/or of the othersubstrate may be exposed to plasma based on oxygen, nitrogen, argon,etc. Nevertheless, for molecular bonding of a sapphire substrate, it ispreferable to use a plasma based on oxygen, since that makes it possibleto obtain a bonding energy that is greater and with a density of defectsthat is smaller in comparison with a plasma based on nitrogen, forexample.

The other parameters or conditions for plasma generation are thosegenerally used by the person skilled in the art. By way of example, theplasma based on oxygen may be generated in equipment originally providedfor performing reactive ion etching (RIE) with capacitive coupling andunder the following conditions:

substrate support chuck connected to a radiofrequency (RE) source at13.56 megahertz (MHz);

working pressure for the O₂ gas lying in the range 20 millitorr (mTorr)to 100 mTorr;

flow rate of the O₂ gas equal to 75 standard cubic centimeters perminute (sccm); and

plasma exposure time lying in the range 10 seconds (s) to 60 s.

Other equipment using an atmospheric plasma, or indeed provided with anelectron cyclotron resonance (ECR) type source or with a Helicon typesource can also be used.

The table below shows the roughness and the contact angle measured atthe surfaces of sapphire substrates and of silicon substrates.

RMS surface roughness Surface (nm) Contact angle (°) preparation Al₂O₃Si Al₂O₃ Si None 0.18 ~0.15 >20 >10 RCA 0.18 0.12 6 <2 cleaning RCA 0.20.12 2 <2 cleaning + O₂ plasma

It can be seen that when the sapphire substrate has been treated with aplasma based on oxygen, its surface presents a contact angle of 2°. Whenthe sapphire surface has not been treated or has been subjected only toRCA cleaning, the contact angle is respectively greater than 20° orequal to 6°. However, when it is desired to perform hydrophilicmolecular bonding, i.e. the type of bonding that is in the mostwidespread use in silicon-on-insulator (SOI) technology, the bondingsurfaces need to present a contact angle of less than 5° in order tohave good control over the quality of bonding.

It may also be observed that the oxygen-based plasma treatment of theinvention does not significantly increase the roughness of the sapphiresurface.

Nevertheless, fabricating a heterostructure of the invention is notrestricted to using hydrophilic bonding. The bonding may equally well behydrophobic.

Furthermore, molecular bonding between the first substrate made ofsapphire and the second substrate having a coefficient of thermalexpansion different from that of the first substrate is preferablyperformed at ambient temperature, i.e. at room temperature without usingmeans for heating the substrate during bonding (a temperature generallyof about 20° C. and that can vary (±10° C.) depending on the temperatureof the room).

There follows a description with reference to FIGS. 3A to 3F and 4 of amethod of fabricating an SOS type heterostructure from a first substrateor initial substrate 110 (Top) and a second substrate or supportsubstrate 120 (Base).

As shown in FIG. 3B, the initial substrate 110 is constituted by an SOItype structure comprising a silicon layer 111 on a support 113 that isalso made of silicon, with a buried oxide layer 112, e.g. made of SiO₂,being disposed between the layer 111 and the support 113.

The first substrate or initial substrate may also be constituted by asimple silicon wafer optionally including an oxide layer on its bondingsurface.

The support substrate 120 is constituted by a sapphire wafer (FIG. 3A).

Before proceeding with bonding the initial substrate 110 on the supportsubstrate 120, the bonding surface 120 a of the sapphire supportsubstrate, which surface has previously been polished, typically by CMP,is itself prepared. In accordance with the invention, the sapphiresubstrate 120 is subjected to stoving, performed in this example at atemperature of 200° C. for a period of 2 h (step S1). As mentionedabove, this staving serves in particular to eliminate contaminants oforganic origin present on the bonding surface of the sapphire substrate,thereby enhancing molecular bonding while limiting the appearance ofdefects.

The bonding surface of the sapphire substrate 120 is then subjected towet chemical cleaning (step S2). The wet cleaning may be performed inparticular by RCA cleaning (i.e. a combination of a bath of SC1 (NH₄OH,H₂O₂, H₂O) suitable for removing particles and hydrocarbons, and a bathof SC2 (HCl, H₂O₂, H₂O) suitable for removing metallic contaminants),cleaning of the “Caro's” or “Piranhaclean” type (H₂SO₄:H₂O₂), or indeedcleaning with an ozone/water (O₃/H₂O) solution.

In order to further decrease the bonding energy, the surface 120 a ofthe substrate 120 can be activated by plasma treatment (step S3). Thesurface 120 a is preferably exposed to an oxygen-based plasma with amean power density that does not exceed 1 W/cm². The other conditions ofthe plasma treatment may correspond to those described above.

The surface 111 a of the silicon layer 111 of the initial substrate 110may be covered in a thermal oxide layer 114, e.g. formed by oxidizingthe surface of the substrate (FIG. 3C, step S4).

The surface 111 a of the initial substrate 110 optionally covered inanother oxide layer, may also be activated by plasma treatment (stepS5). Since this is a silicon surface, it may be exposed to a standardplasma, i.e. a plasma based on oxygen, nitrogen, argon, etc., with powerdensity that is not limited to 1 W/cm². Activating a silicon bondingsurface is well known to the person skilled in the art and is notdescribed in greater detail for reasons of simplification.

One or more cleans subsequent to the plasma exposure may be performed,in particular in order to remove the contaminants that were introducedduring exposure, such as rinsing in water and/or cleaning in SC1 (NH₄OH,H₂O₂, H₂O), optionally followed by drying by centrifuging. Nevertheless,these cleans may be replaced by scrubbing that enables a large fractionof the contaminants to be eliminated.

Once they have been prepared, the surfaces 111 a and 120 a are put intointimate contact and pressure is applied to one of the two substrates soas to initiate the propagation of a bonding wave between the contactingsurfaces (step S6, FIG. 3D).

The bonding is then reinforced by performing a bonding reinforcing orstabilizing anneal (step S7). As mentioned above, because of thedifference between the coefficients of thermal expansion betweensapphire and silicon, the stabilizing anneal is performed at atemperature of less than 300° C. By way of example, the stabilizinganneal may be performed at a temperature of 180° C. for a duration of 2h.

The fabrication of the heterostructure is continued by thinning theinitial substrate 110 so as to form a transferred layer 115corresponding to a fraction of the silicon layer 111 (step S8, FIG. 3E).Thinning is performed initially by grinding off a major fraction of thesupport 113 and is then continued by chemical etching, e.g. by means ofa solution of tetramethylammonium hydroxide (TMAH).

In an optional step, the structure is edged so as to remove the chamfersand edge roll-off present at the peripheries of the substrates (step S9,FIG. 3F). As shown in FIG. 3F, this gives rise to a heterostructure 200comprising the sapphire support substrate 120 and the transferred layer115, with an interposed buried oxide layer 114.

FIG. 5A shows an SOS type heterostructure obtained from an initial SOIsubstrate bonded on a sapphire support substrate. Prior to bonding, thebonding surface of the sapphire substrate was prepared using RCAcleaning and scrubbing. After bonding, the structure was subjected to astabilizing anneal at 200° C. for 2 h and it was thinned by grinding andby chemical etching with TMAH.

FIG. 5B also shows an SOS type heterostructure that was made differentlyfrom that of FIG. 5A in that prior to the RCA cleaning and thescrubbing, the bonding surface of the sapphire substrate was alsoprepared by:

stoving at 200° C. for 2 hours;

RCA cleaning (O₃/H₂O, SC1 (NH₄OH, H₂O₂, H₂O), and SC2 (HCl, H₂O₂, H₂O));and

oxygen-based plasma activation with a mean power density not exceeding 1W/cm².

In FIG. 5B, it can be seen that practically no defect is visible in thetransferred silicon layer, whereas in FIG. 5A, numerous defects arepresent at the bonding interface and also in the transferred siliconsupport. These figures thus demonstrate the combined effect of stovingand of surface activation by plasma treatment on reducing the defectspresent after bonding and a stabilizing anneal.

As explained above, the stoving step of the invention makes it possibleto increase the bonding energy in an SOS type structure. This bondingenergy may also be increased by activating the bonding surface of thesapphire substrate by plasma treatment as described above. As shown inFIG. 1, it can be seen that the bonding energy is even greater when thesurface of the sapphire substrate has been exposed, after stoving, RCAcleaning and scrubbing, to a plasma (column D) as compared with noplasma treatment (column C).

The invention may also be applied to layer transfer techniques otherthan that described, e.g. in application of the Smart Cut technology.

1. A method of fabricating a heterostructure comprising at least a firstsubstrate made of sapphire and a second substrate made of a materialhaving a coefficient of thermal expansion that is different from that ofthe first substrate, the method including a step of molecular bondingthe second substrate on the first substrate made of sapphire, and priorto bonding the two substrates together, a step of stoving the firstsubstrate that is performed at a temperature lying in the range of 100°C. to 500° C., and when performed at a temperature in the range of 100°C. to 200° C., the duration of the stoving step is at least 1 h.
 2. Themethod according to claim 1, that wherein the staving step is performedat a temperature of about 200° C. over a duration of about 2 h.
 3. Themethod according to claim 1, wherein the stoving step is performed underan atmosphere of air or of inert gas.
 4. The method according to claim1, comprising, after the stoving step, a wet chemical cleaning step. 5.The method according to claim 1, comprising, prior to bonding the twosubstrates together, a step of activating the bonding surface of thefirst substrate made of sapphire by plasma treatment, the mean powerdensity of the plasma used being less than or equal to 1 W/cm².
 6. Themethod according to claim 5, wherein the bonding surface of the firstsubstrate made of sapphire is exposed to a plasma based on oxygen. 7.The method according to claim 1, comprising, prior to bonding the twosubstrates together, forming an oxide layer on the bonding surface ofthe second substrate.
 8. The method according to claim 1, comprising,prior to bonding the two substrates together, a step of activating thebonding surface of the second substrate by plasma treatment.
 9. Themethod according to claim 1, comprising after the two substrates havebeen bonded together, a step of stabilizing the bonding by annealing ata temperature of less than 300° C.
 10. The method according to claim 1,wherein the second substrate is constituted by a layer of silicon. 11.The method according to claim 1, wherein the second substrate isconstituted by an SOI structure.
 12. The method according to claim 1,wherein the step of molecular bonding the second substrate on the firstsubstrate made of sapphire is performed at ambient temperature.